Frequency spacing in two-tone carrier system



A ril 20, 1954 w. A. PHELPS 2,676,203

FREQUENCY SPACING IN TWO-TONE CARRIER SYSTEMS Filed Sept. 1, 1950 4Sheets-$heet 1 FIG.

MARKING Moauu 1m MARKING CHANNIEL F/L m? TRANSMITTER F ROM OTHER 23CHANNELS MA RK lNG' 05 C /L LA TOR SPAC/NG OSCILLATOR I \SPAC/NG CHANNELFILTER MARK AMPL/ TUDE SPACE INVENTOR By m4, PHELPS ATTORNEY April 20,1954 w. A. PHELPS 2,676,203

FREQUENCY SPACING INTWO-TONE CARRIER SYSTEMS Filed Sept. 1, 1950451168tS-Sh66t 2 A ril 20, 1954 w. A. PHELPS 2,576,203

FREQUENCY SPACING IN TWO-TONE CARRIER SYSTEMS Filed Sept. 1, 1950 4Sheets-$heet4 FIG. 5A

5 a 3% 5 w a & N SIG/VAL BANDS PASSED '---1 BY CHANNEL FILTERS A B :0 .05 i 50- i l c l I E 40-. I a 3"- 5 l a, 20-

l0 SIGNAL BAND 0 l l I I l I l FREOUENCV- crass PER SECOND PRODUCTS 0FSEL ECTIV/TV CHARACTERISTICS MODULATION 0F DISCR/M/NA TOR FILTERS W LWITH? (a 3 ORDER MODULATION nooucrs 3 ORDER MODULATION moouc rs Q \r w ID V I k "i Q "5 N V E 3 w I l E q {L 9 F a 20 5 E as, u E a Q n p l v u,o I I MARK I $PA|CE l a 7 I FREOUENLY- CYCLES PER SECOND FIG. 5C

(SIG/ML FREQUENCIES DEMODUMTED BANDS 0 so 100 msounvcrcra ES PER sc0-0INVENTOR By M. A. PHELPS AT TORNEV Patented Apr. 20, 1954 FREQUENCYSPACING [IN 3 TWO-:TONE' CARRIER SYSTEMS Walter A. Phelps, Madison,N.J,,assignor,toBell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application September 1, 1951),=Serial No.182,650:

This invention relatesto carrier-communication systems and, moreparticularly, to a frequency allocation plan for a two-tonev carriertelegraph systemusing different frequencies for marking and spacingsignals.

A principal objeotof the invention is to improve the signal transmissionquality of a twotone; carrier telegraph system.

It is sometimes desirable to use a two-tone carrier system .inpreference to a true, frequency shift system. For example, itmay bedesired to convert an existing single tone amplitude, modulated or.on-ofi carrier telegraph system to twotone operation. In the past,ithas generally been thought. that the signaltransmission quality of atwo-tone systemis inherently poorer than the quality of a true frequencyshift system. Eiiorts to ,reduce the signal distortion in two-tone,systems have not succeeded toany appreciable extent primarily becausethe cause of this distortion has not been fully appreciated. Applicant,however, has succeeded in recognizingthe causes of this distortion andhas foundthat thetransmissioii'ahd signal quality advantagesof true fre:quency shift operation may be equaled'ina twotone system by a properspacing of the carriers employedfor marking andjspacing signals.v

In accordance with an illustrative embodiment of the invention, to bedescribed later indetail,

two-tone carrier telegraph signals are trans, mitted by twointerconnected, modulators, .marlning and spacing tones beingalternately trans.- mitted under the control of a single sending loop.At the receiver, the marking and spacing tones belonging to one two-tonechannel are selected by filters and passed through a common limiter. Themarking and spacing tones are then separated by filters and passedthrough separate 1amplifier-detectors; 'Iheoutputs 'of these detectorsmay be combined difierentially through a relay or other load circuit, orthey may be made to operate a vacuum tube load circuit in such a-manneras to produce full marking current when a marking tone is received-or nocurrent when a spacing signal is received, an operating conditionnormally required for teletypewriters. A circuit designed to operate inthe latter manner is disclosed in a copendingapplication of T. A. Jones,Serial No. 181,951, filed August 29; 1950, now- Fat-- @111; NO.

A featureotthe invention .is thatwhile .two-. tone systems, as -.usually.constructed, with separate oscillator supplies. produce signals ofv,high distortion, the signal ,distortion can be reduced to a point whereitis comparable with ,thatproduced in true,frequency-shiitsystems byproperly selecting the frequencies-toJoe-paired .in onechannel instead.of merely using. adjacent, available. f1'equeneies,,as has.generzillybeen .the practice.

This and other featuresandobieotsoi the. ine vention maybe better.understood,..fromascensideration.v of the following detailed descriptionwheniread in accordancewith the. attached drawings and tables, in which:

Fig..- 1 shows.,tr-ansmitting!circuits for sending two-tone carriersignals;

Fig. 2 shows the corresponding receiving ,cir cuits;

Fig. 3 shows theenvelopes, ofsignalsbeforeand after passing ,through the,limiter Fig. .4 .shOWS the distributionof signal ,sideband frequenciesFigs. ,5A; through I 50 show, theihistoryoi mark and space signal.bands, in a tvvortone system using carriers of 12 125 cyclesand229'5cycles; and

Fig. Gshows the history, oimark andspace sig-. nal bands 7 in. antwo-tone receiving. circuitusingcarriers of, i 1955' cycles and, 229.5-cycles.

In the discussion which follows, .itis assumed that each markingirequencyiand each spacing frequency, is obtained from a. separatevacuum tube oscillaton, These oscillators areituned to emit frequencies.which are odd1mu'1tiples of. a base frequency, since, as} is known,certain .advantages result from theuse of frequencies which are sorelated! Specifically, thesefrequencies .ex-

tend from.42.5cycles to 2295, cycles, inclusive, .at 170"-o ycleintervals and are theiodd multiples of 85 :cycles beginning withthe[5th. The operation of the system .described ,isnot dependent. .on theseparticular numerical, values these. have. been chosen. asrepresentative, and a. considerable number of measurements. and.calculations ,have been madeusing suchanarrangement. Itis alsoassumedthat these oscillators .are'locked through their gridjcircuitstoacommonbase. frequency oscillator (85" cycles in. the illustration used hr in) in der. thatall frequenciessused may be exact multiples, ofa.commombasecfrequency. This, provision is not essentialfto theoperation of 3 the system but yields an additional advantage which willbe discussed later.

Referring now to Fig. 1, the essential principles of the sendingmodulator will be described. A sending device, such as a telegraph keyor a teletypewriter sending keyboard, is represented by theopen-and-close contact II. When the key is closed, the resistance bridgeI2 is unbalanced in such a direction as to apply a positive potential topoints I3 and I4 of the marking modulator I5 and negative potential topoint I6. This will cause varistors I1 and I8 to have low impedance andvaristors I9 and 20 to have high impedance. These conditions will allowcarrier current from the marking oscillator 2| to flow outward, withlittle attenuation, through the marking channel filter 22 to the radiotransmitter 23. Meanwhile, a positive potential will be applied to point24 of the spacing modulator 25 and a negative potential to points 25 and21. This will cause varistors 28 and 29 to have high impedance andvaristors 30 and SI to have low impedance. These conditions willefiectively block the fiow of spacing carrier current from the spacingoscillator 32. When the contact at key I I is opened, all the conditionsdescribed above are reversed; the bridge I2 is unbalanced in theopposite direction, and all the potentials referred to above arereversed. With this condition, only spacing carrier current is allowedto flow through the spacing channel filter 33 to the radio transmitter23. Thus, as key contact I I is alternately opened and closed, markingand spacing carrier current are alternately released to the radiotransmitter 23, thereby transmitting the desired telegraph signals.

Referring next to Fig. 2, the principles of the receiving circuits willbe described. Signals comprising marking and spacing carrier currentsfrom the radio receiver M are selected by the marking channel filter 42and by the spacing channel filter 43, respectively. These currents thenpass through a common current limiter 44, which may, for example, be ofthe type disclosed in E. Peterson Patent 1,830,240, dated Novemher 3,1931. The telegraphsignals, in passing through the channel filters 42and 43, have been considerably rounded. The limiter 44 transforms theminto square signals, and it is this property of the limiter which givesrise to signal distortion in two-tone systems using separate carriersupply oscillators for marking and spacing tones. This matter will bediscussed more in detail below.

The marking and spacing signal currents are again separated by means'ofthe mark discriminator filter 45 and by the space discriminator filter46, respectively. These signal currents are then separately amplified bythe amplifiers 47 and 48 and rectified by the varistor rectifiers 49 and50. Following each rectifier is a low-pass filter, 5I and 52, to passthe signal frequencies and to suppress the'carrier and any of itscomponents passed by its associated rectifiers 49 or 58. The signalcurrents then pass to the marking direct-current amplifier 53 and to thespacing direct-current amplifier 54, respectively. Under the influenceof the negative direct-current potential supplies 55 and 56, tubes 53and 54 are normally operated below cut-off so that with no incomingspace or mark signal current, no current will flow in the plate circuitsof these tubes. With incoming marking current,. the negative grid biasof tube 53 will be overcome, and current will flow in the plate circuitof the marking direct-currentamplifier tube. Similarly, upon the receiptof a spacing signal, the marking directcurrent amplifier tube 53 will becut off, and current will flow in the plate circuit of the spacingdirect-current amplifier tube 54. It can be seen from Fig. 2 that whencurrent flows in the marking tube and then in the space tube, a currentwill flow through the receiving load circuit 51 first in one directionand then in the other. This load circuit may, for example, be the relaywinding or magnet coil of a .teletypewriter or other electromagneticallyoperated recording or indicating device.

The description of the receiving circuit just given covers polaroperation only, and this case has been used to establish the generalprinciples of receiving circuit operation, as it is the simplest case.It is also possible to arrange the receiving circuit so that it willtransmit to the receiving load circuit neutral signals, that is, currentand no current to distinguish between marking and spacing signals, asdescribed in the above-mentioned application of T. A. Jones. However,the principle of two-tone operation is not aifected by the type ofsignal to be transmitted to the receiving load circuit.

Now that the principles of operation of the sending andreceivingcircuits have been set forth, the course of the telegraphsignals through the receiving circuit will next be considered. Supposethe sending circuits of Fig. 1 are transmitting a stream of equallyspaced dots. After pass-' ing over the circuit to the receiver in Fig. 2and through the channel receiving filters, these dot signals willresemble in form those indicated in Fig. 3, lines A and B. Those in lineA represent the dots in the marking channel, and those in line 13represent the dots in the spacing channel. It is to be understood thatthese diagrams are amplitude-time representations of the signals. Therounded form of the carrier envelope is caused by the subtraction by thefilters of the odd order components present in the original squaresignals.

When these signals have passed through the current limiter 44 in Fig. 2,they may be represented, on anamplitude-time basis, as in line C of Fig.3. The effect of the limiter has been to square up the signals; that is,the odd order harmonics of the signal frequencies subtracted This willapproximately represent the marking .dots of line C in Fig. 3 and alsothe spacing dots of line C in Fig. 3, the only difference between thetwo being the angular velocity or of the carrier frequency (differentcarrier frequencies are used in the mark and space channels). The re1-act-apes ative amplitudes: of the first few' components in themodulated-wave are set forth in Table I- below.

. Suppose I we-haye 1 a -two,-tcne channel in' which the ma kin f cq encs.2.l 5 c l a the spacing frequency 13,2295; cycles and; examine theeffect of the sidebands associatedwith'a-spacing -signal. Thesituationis ,asashownin Fig.4, in ,which'Fl represents-{thediscrimination-frequency characteristic of ,the, space discriminatorfilter .iollowing the, liniiterand R2 represents the similarcharacteristieof .themark discriminator, filter.

the signal sidebands of the.spacing signals will be as represented-inFig.- 4. by. the numbered vertical lines. Cris thecarrier currentat themidfrequency of the ,filten-passband the magnitude .,of. Chi-Sarbitrary. 'Ihe first. order signalside- .band on .each side orCisrepresented by the .yertical line; carrying the number I and is; 3.9decibels-below O in magnitude and occursv at 37 .cycles. on either. sideof-C. The thirdorder com- .ponent occurs, at,3 37.,cyc1es-.on eithersideof C,

- proximately- '13 decibelsby:the'space='discrimina-= 1 torfilter sothatit is not 'very efiectivwin determining the: shape of "thespacingsignal. I How'- ever, it suffers only about: 3 decibels: attenuation 5by the mark discriminator if filter. The:*tl1-ird order component-01Ethe spacing signal isetherefore, more-effectivein shaping theimarkingsignal than the spacing signal. -However;- --its: pha-se re- -lation tothemarking carrier is generallyrwrong for-properly shaping thelatterbecause the spacing and marking carrier currents arederivedrromdiflferent oscillators. Consequently, the: third .harmonic of thespacing signal frequency. will operate generally todistort J'the shape==of wthe marking signal. The same-is true of the-= fiith order componentof the spacing signal,--which,..as can be seen fr0m Fig. 4,-issubstantially =unattenuated by theumark discriminator filter. In

- evaluating this effect, it is important to understand that during thetransitionfro'm =mark to space, or from space to mark, bothsignals arepresent simultaneously; andit -is'atthis transition time that anydistorting eifecton the signal shape is most troublesome because it isat this time that-the load circuit operatesfrcm space If, instead-ofusing a filter having the discriminator characteristic'Fz and amid-bandfrequency of 2125 cycles as#amark discriminaton filter (5 v in Fig. 2),a filter having the-discriminator='characteristic Fmand mid-bandfrequency-M 1955 cycles is used, it will-be noticedbyinspection'pf,Table II Channel Freq, Max Channel Freq, Max .OhannelrEreq, ,C. P. S.Distob C. P. S. Distal, n .C. P. S. H ,Distop tion, ,,tion, .ition, MarkSpace percent Mark Space Mark Space percent '425 595 23 765 1,445 817275 1', 785 6 .425 765 a 10 765 I 1,615; 8 1,275 51. 955 i 7 425 935 9765 1, 785 6 1, 275 2, 125 5 425 1,105 12 765 1,955 8 1, 275 2, 295 6425 1, 275. 7 J 765 2,125 3 11. .1

425 1, 445 13 765 2,295 6 ,1, 445 1, 615 .18 425 1, 615 8 1;445 1; 7857. 5 425 1, 785 8 935 1,105 .19 1,l445j '1, 955 5 .425 1,955 9 935 1,275 8 ,..1,,445 2,125 5 425 2,125 6 935 1, 445 6 1,445 2,295 6 425 2,295 9. 5 935 1,515 i 7- j 935 1, 785 7 .1,.615 1, 785 .120 595 765 29935" 1", 955 '8 1,615 1,955 5 595 935 7. 5 935 2, 125 5 1, 615,v 2,125 i4; 5 595 1, 9 935 2,295 a 6 1,.615 2,295, 4

595 1, 785 7 1,105 I 1, 515 5 1,785 2,295 4.5 V 595 1, 955 13 1, 105 1,.785 7 595 2,125 7. 5 1,105 1,955 7 1,955 2,125 18 595 2, 295 B 1,-1052, 125 7. 5 1,955 2,295- i 6 1,105 7 2,295 as. 5 r 765 935 18 2,125 2,29s .15 765 1, 105 8 1, 275 1,445 20 765 1, 275 6 1, 275 1,-615 7 anditsmagnitude in each case is 13.5 decibels :ipqnentrof ;th asnaoih sign lciattenuated ap- Inspection ofthis, table, shows, thatzwithga spacingcarrier freque1 1cy:of-2295; cycles and with; a

marking frequency 0f:;2l25;cycles: ,(11 liner-in I thetable).Aha-observed:signaLdist rtiomwas 16 7 last line) a distortion ofonly 6 per cent, which illustrates experimentally the advantage citedabove. So far, only the cifect of the sidebands 'on the spacing signalin distorting the marking uses a 7.41 unit code, and at a signalingspeed of 100 words per minute, themaximum signaling frequency is 37 dotcycles per second. Of course, the transmission of miscellaneous signalsof ordinary trafiic does not consist of a stream of equally spaced dotsand hence does not present a spectrum of signal frequencies like thatshown in Fig. 4. The stream of equally spaced dots has been used in thisdiscussion because it is the severest case; that is, the signaldistortion is greatest in this case. The measurements tabulated in TableII were made with the transmission of miscellaneous signals such as areuse in ordinary telegraph traflic.

In addition to the direct spill-over" of signal sideband into a closelyspaced adjacent channel, there is a secondary efiect due to theintermodulation of signal sidebands. Reference to Figs.

1 and 2 show that miscellaneous signals modulated on the carrier usedpass through both a send-channel filter (22 or 33 in Fig. 1) and areceive-channel filter (42 or 43 in Fig. 2). A typical selectivitycharacteristic of two such filters added together is shown in Fig. 5A.On this characteristic at the IO-decibel point is shown the spacingcarrier (2295 cycles) 1*: all frequencies up to the third harmonic of 37cycles. This desirable band of signal frequencies is indicated by theline any. The IO-decibel point on the filter characteristic has beenchosen because modulation components produced by currents attenuateddecibels can, in general, be neglected in this study. The part of thisband of frequencies which passes through the combined channel filters isindicated in the figure at CD.

The marking carrier (2125 cycles) with its signal sidebands similarlypasses through two channel filters inseries and emerges as the line ABin Fig. 5A. The shape of the send and receive filters in the 2125-cyclemarking path have been I assumed the same as in the 2295-cycle spacingpath; this is approximately true in the practical case.

At a time of transition from mark to space or space to mark, in thetwo-tone system, both marking and spacing currents are presentsimultaneously in the current limiter. As the limiter is a strongmodulator, it is important to examinewhat products can pass through thediscriminator filters (45 and 46 in Fig. 2)

When the bands AB and CD pass through a modulator producing asymmetrical output wave, the products obtained are as shown in Fig. 5B.These extend from m to n and from p to q. Every frequency from m to nand p to q does not appear, but discrete frequencies will be distributedat intervals between these limits. The lines indicating the modulationproducts have been drawn in arbitrarily at the lo-decibel point on themark and space discriminator filters, but this is not to be taken tomean that this is an indication of their magnitude.

It can be seen that some third order products fall inside the pass bandsof both discriminator filters and enter the demodulator. 0ndemodulation, the situation is as' shown in Fig. 5C. The important pointis that the interfering band resulting from intermodulation of signalsidebands in the limiter completely overlaps the signal band; and as thephase relations between the two bands are random, they will, in general,beat with each other during a signal transition, causing distortion.

If the marking frequency is changed from 2125 cycles to 1955 cycles,leaving the spacing frequency at 2295 cycles, the products resultingfrom the intermodulation of sidebands will change their positions in thefrequency spectrum. Again tracing the history of the signal bands in thereceiving circuit, we find the situation as shown in Fig. 6. Now, nomodulation components produced in the limiter, of the fifth order orbelow, lie within the bands of the discriminator filters below the pointof 20 decibels discrimination. By shifting the marking frequency from2125 cycles to 1955 cycles, not only has the direct leakage of sidebandsfrom one channel to the other been avoided, as pointed out previously,but also the products of modulation of the signal sidebands in thelimiter have been thrown outside the bands of the discriminator filters(45 and 46 in Fig. 2).

In the specific case just considered, the predominant efiect is thedirect overlap of signal sidebands first considered above, but theeffect of intermodulation of sidebands in the absence of any appreciableoverlap of sidebands can be seen by reference to Table II previouslyconsidered. An examination of this table shows that occasionally wherethe mark and space frequencies are so far apart that any appreciableefiect from the direct overlap of sidebands is unlikely, the signaldistortion will suddenly rise appreciably. For example, in the group ofchannel pairings of Table 11 above, using 765 cycles as the markingfrequency, the 765-2125 cycle pair shows a marked rise in measureddistortion, although these two frequencies are to'ofar apart for anydirect overlap of sidebands to occur.

The entire group of pairings, using 765 cycles as the marking frequency,were analyzed, using the method set forth on Fig. 6. The results of thisanalysis are shown in Table III below. It will be noted that even orderproducts have been neglected. This has been done because the limiter hasbeen so designated that with a sine wave input, the output issubstantially a symmetrical square wave. Such a device will producemainly odd order products of modulation.

An inspection of Table III shows high. signal distortion where the markand space frequencies are close together or where low order products ofmodulation between signal sidebands fallin the bands of the mark orspace discriminator filters or in the bands of both filters.

Looking at the last line of Table III and noticing that a third orderproduct falls in the band of the mark discriminator filter and that afifth order product falls in the band of the space discriminator filter,it would be reasonable to expect a higher distortion than 6 per cent. Inthis case, it will be observed that the two frequencies of the channelpair are exact multiple of each other; and the products of modulationfrom the limiter, if they lie in the pass bands of the discriminatorfilters, will have exactly the same frequency as the signal componentswhen demodulated. These products of modulation will not necessarily beexactly in phase with the signal components, but whatever the phasediiference may be, it will not change because the carrier oscillatorsare locked together by means of an 85-cycle base frequency oscillator,as previously mentioned. Therefore, there will be no beat between thedemodulated signal frequencies and the demodulated products. These mayshorten or lengthen both the marking and spacing signal, which mayproduce a bias, but this can be adjusted out and will not appear as avarying distortion of the signal.

If the oscillators are unlocked and allowed to drift apart slightly, thedistortion will rise; measurements showed a rise of 3 or 4 per cent indistortion, but the oscillators used normally produce frequencies closeto the desired value. For a considerable difference in frequency, therise of distortion would have been larger.

From the analysis given above, supported by experimental data, itappears that given a group of carrier frequencies bearing an oddharmonic relationship to a common base frequency, a two-tone telegraphsystem may be constructed,

using separate sources of carrier current for mark and spacefrequencies, which will show satisfactorily low signal distortion if themark and space frequencies are so chosen that they are separated by aninterval sufiicient to prevent appreciable direct overlap of sidebandsand so that third order products of modulation of the signal sidebandsemerging from the limiter lie outside the bands of the discriminatorfilters placed between the limiter and the demodulator. This assumesthat the limiter is a symmetrical device so that substantially only oddorder products of modulation are produced. However, if the mark andspace frequencies are exact odd multiples of each other and are held instep with each other by an automatic lock-in device, the distortion(other than a constant bias) will still be low in spite of anymodulation products reaching the demodulator so long as the mark andspace frequencies are sufficiently separated to avoid direct sidebandoverlap.

Although the invention has been described with reference to particularembodiments and numerical values, it is to be understood that theinvention is not limited to these specific embodiments or values, sinceother embodiments and modifications will readily occur to one skilled inthe art.

What is claimed is:

1. In a two-tone carrier communication system means comprising a firstcarrier source for transmitting marking signals, a second carrier sourcefor transmitting spacing signals, and a receiver including a commoncurrent limiter for limiting said carriers and filtering means forseparating said marking and spacing signals at the output of saidlimiter, said first and second carriers having frequencies bearing anodd harmonic relation to a common base frequency and being separated infrequency so that marking and spacing signal sidebands associated witheither carrier at the output of said limiter will be sufficientlyattenuated by the filtering means designed to accept the spacing andmarking signals, respectively so as to have an unappreciahle effect onthe shaping of the spacing and marking signals, respectively.

2. The combination in accordance with claim 1 wherein said filteringmeans comprise means for passing said marking and spacing signals andmeans for suppressing third order modulation products arising fromintermodulation in said limiter of said marking and spacing signals.

3. In a two-tone carrier system employing different frequencies forspacing and marking signals and employing filters for separating themarking signals from the spacing signals, the method comprising pairinga marking frequency with a spacing frequency sufficiently separated fromsaid marking frequency to prevent the sidebands representing the markingand spacing signals from overlapping the pass-bands of the spacing andmarking filters, respectively, to an appreciable extent and to preventthird order modulation products resulting from intermodulation of thesignal sidebands from overlapping the pass-band of either of saidfilters to an appreciable extent.

4. In a two-tone carrier communication system employing a markingcarrier of a first frequency for mark signals and a spacing carrier of asecond frequency for space signals, means for transmitting said mark andspace signals as sidebands of their respective carriers, a receiverincluding means for receiving said mark and space signals, a commoncurrent limiter, frequency discriminator means having pass bands whichembrace said mark and space signals connected to the output of saidlimiter and means for applying the received mark and space signals tosaid limiter whereby the output of said limiter includes the limitedmark and space signals and distortion in the form of modulation productsresulting from the intermodulation in said limiter of said mark andspace signals, said first and second frequency being odd harmonics of acommon base frequency and said frequency discriminator means comprisinga first filter having a pass band region embracing said mark signals tothe exclusion of said space signals and a second filter having a passband region embracing said space signals to the exclusion of said marksignals.

5. The combination in accordance with claim 4 wherein said carriers areseparated in frequency with respect to the pass band regions of saidfilters to throw said modulation products which are of the third orderoutside said pass band regions.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,154,921 Vroom Apr. 18, 1939 2,370,985 Morrison Mar. 6, 19452,406,034 Phelps Aug. 20, 1946 2,461,956 Beckwith Feb. 15, 19492,477,963 Chapin Aug. 2, 1949

